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A highsider or highside is a type of motorcycle accident characterized by sudden and violent rotation of the bike around its long axis. This generally happens when the rear wheel loses traction, skids, and then suddenly regains traction, creating a large torque which flips the rider head first off the road. The initial traction loss may be caused by a rear locked wheel due to excessive braking or by applying too much throttle when exiting a corner or by oversteering the bike in the turn or by any loss of traction to the rear wheel. Examples are: attempting to turn too sharply and leaning the bike past the limit of tire adhesion; rolling off the edge of the tire sidewall; levering the rear tire off the ground by scraping the exhaust, peg, leg, foot, or knee; or overstressing the rear tire if it's too cold or worn-out or there is oil, water, sand, dirt, ice, paint marks, or other patches of reduced traction on the road. Highsides differ from lowsides as follows: during a lowside the rear wheel slips laterally and continuously until the bike falls onto its side -- the side that's inside the corner, while during a highside the rear wheel slips laterally only briefly before suddenly regaining traction and flipping the bike onto its other side -- the side that's outside the corner. As a result highsides happen very quickly with little if any warning and are very violent.
If the slipping rear tire suddenly regains traction while the bike is side-slipping, the motorcycle will straighten-up very quickly. Often the rider is thrown off before they can regain balance and control.
Highside accidents can be caused by over-acceleration or braking in corners, but also can result from any sudden loss and regaining of traction to the rear wheel. In all cases, the rear-wheel begins to move out of the line of travel and side-slips. If the wheels are not aligned in the direction of travel when traction is suddenly restored and the rear-tire stops slipping then a highside is likely, depending on how much the bike is turned across the direction of travel and how fast the bike is traveling when the rear tire stops slipping. If the angle is high enough, the bike is moving fast enough and the rear tire slips and hooks-up suddenly enough, the rider has no chance of preventing a highside and not taking a short flight across hard ground. What makes the highside so deadly is that the bike can go from traveling down the road at high speed completely under control to tumbling down the road at high speed with the rider flying in front of it, all within a split second with little if any warning.
Forces occurring between the motorcycle and the road (such as those that result in accelerating, decelerating and turning) occur at the contact patch through friction and normal forces. There is a limited amount of force tangential to the road that the contact patch can transmit before the tire begins to lose traction, and therefore slide or skid.
When going through a curve on a motorcycle, centripetal force (added to the other lateral forces such as acceleration or deceleration) is transferred from the road to the motorcycle through the contact patch, and is directed at a right angle to the path of travel. If the net force is greater than the static friction coefficient of the tire multiplied by the normal force of the motorcycle through the tire, the tire will skid outwards from the direction of the curve.
Once a tire slips in a curve, it will move outwards under the motorcycle. What happens from there depends on how well the rider is able to restore balance and control. If the tire regains traction after the rider starts to skid while the motorcycle is moving sideways, the tire will stop its sideways movement causing the motorcycle to suddenly jerk into an upright position (and beyond). This movement can easily cause the rider to be thrown off and even expert riders are routinely thrown-off in highsides.
The name ("highside") derives from the side of the motorcycle that the rider will separate from. If forcibly thrown over the bike, the rider is said to have dismounted on the high side.
The highsider has the additional disadvantage of the rider often being catapulted quite some feet into the air by the sudden jerking motion of the motorbike and the increased possibility of damage upon impact with the ground as well as being hit by the motorbike that is tumbling, bouncing and sliding behind the rider, threatening to crush them. It is somewhat like being thrown from a horse at full-gallop, except that a motorcycle can travel much faster than a horse and is generally ridden on hard-paved roads with plenty of roadside obstacles.
Because highsider accidents are so much more deadly than lowside accidents, the Motorcycle Safety Foundation recommends that if a rider locks the rear brake at higher speeds and the traction is good the brake should not be released.
- "You and your motorcycle: riding tips" (PDF). Motorcycle Safety Foundation. p. 36. Retrieved 2009-09-28.